1+ < h1 > Well, In This Case...</ h1 >
2+ < table class ="table meta-table table-condensed ">
3+ < tbody >
4+ < tr >
5+ < td class ="header-column "> < strong > Contributed by</ strong > </ td >
6+ < td > Andrew Phillips</ td >
7+ </ tr >
8+ < tr >
9+ < td > < strong > Source</ strong > </ td >
10+ < td > < a target ="_blank " href ="https://groups.google.com/d/topic/scala-language/DDpvGcrs3QQ/ "> Didier Dupont </ a > </ td >
11+ </ tr >
12+ < tr >
13+ < td > < strong > First tested with Scala version</ strong > </ td >
14+ < td > 2.11.2</ td >
15+ </ tr >
16+ </ tbody >
17+ </ table >
18+ < div class ="code-snippet ">
19+ < h3 > What is the result of executing the following code?</ h3 >
20+ < pre class ="prettyprint lang-scala ">
21+ case class Toy(squeezeMsg: String = this.toString) {
22+ override def toString = squeezeMsg
23+ }
24+ println(Toy("My name is Fido!") == new Toy("My name is Fido!"))
25+ println(Toy() == new Toy())
26+ </ pre >
27+ < ol >
28+ < li > Fails to compile</ li >
29+ < li id ="correct-answer "> Prints:
30+ < pre class ="prettyprint lang-scala ">
31+ true
32+ false
33+ </ pre >
34+ </ li >
35+ < li > Prints:
36+ < pre class ="prettyprint lang-scala ">
37+ true
38+ true
39+ </ pre >
40+ </ li >
41+ < li > The first statement prints:
42+ < pre class ="prettyprint lang-scala ">
43+ true
44+ </ pre >
45+ and the second throws a runtime exception
46+ </ li >
47+ </ ol >
48+ </ div >
49+ < button id ="show-and-tell " class ="btn btn-primary " href ="# "> Display the correct answer, explanation and comments</ button >
50+ < div id ="explanation " class ="explanation " style ="display:none ">
51+ < h3 > Explanation</ h3 >
52+ < p >
53+ As per the SLS (§5.3.2), if the elements (i.e. constructor parameters) of
54+ a case class have default arguments, these are carried over to the
55+ < tt > apply</ tt > factory method on the companion object that is generated by
56+ the compiler. Also, when < tt > Toy</ tt > is declared, there is indeed a
57+ < tt > this</ tt > reference in scope, so the code compiles successfully.
58+ </ p >
59+ < p >
60+ The < tt > this</ tt > in < tt > this.toString</ tt > (the value of the default argument
61+ for < tt > squeezeMsg</ tt > ) does not refer to the instance of < tt > Toy</ tt > being
62+ created, however. Instead, it resolves to the instance of the class or object
63+ in which < tt > Toy</ tt > 's constructor is defined.
64+ </ p >
65+ < p >
66+ When running the example in the REPL, the class or object invoking the
67+ constructor < tt > new Toy()</ tt > directly is a generated "script object":
68+ < pre class ="prettyprint lang-scala ">
69+ scala> println(new Toy())
70+ $line46.$read$$iw$$iw$@7335aa7c
71+ </ pre >
72+ When creating a new < tt > Toy</ tt > instance using the standard factory method
73+ generated for case classes, however, the class or object invoking < tt > Toy</ tt > 's
74+ constructor is actually the companion object < tt > Toy</ tt > :
75+ < pre class ="prettyprint lang-scala ">
76+ scala> println(Toy())
77+ Toy
78+ </ pre >
79+ </ p >
80+ < p >
81+ Since the generated implementation of < tt > equals</ tt > for case classes
82+ uses the values of the case class's elements (< tt > squeezeMsg</ tt > , in this case)
83+ to determine equality, the two instances < tt > Toy()</ tt > and < tt > new Toy()</ tt >
84+ are, thus, < em > not</ em > equal.
85+ </ p >
86+ </ div >
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